Bay's September Low-Oxygen Zone Ranked Third-Largest in 42 Years; Maryland Sampled Before the Nor'easter

ANNAPOLIS, MARYLAND — September monitoring found the Chesapeake Bay's third-largest volume of low-oxygen water for that month in 42 sampling years, according to results the Maryland Department of Natural Resources released Oct. 19. The low-oxygen water was found in Maryland, whose waters were sampled before a passing nor'easter.

Hypoxic water, which holds less than 2 milligrams of oxygen per liter, measured 1.02 cubic miles on the combined Maryland and Virginia September mainstem cruises, against a 1985–2025 September average of 0.53 cubic miles. Out of 42 sampling years, only two had a larger September hypoxic volume. DNR ranked the cruise 40th of 42, with the ranking running from the smallest volume, or best conditions, to the largest.

The most oxygen-starved water grew slightly. September held 0.30 cubic miles of anoxia, water with less than 0.2 milligrams of oxygen per liter. DNR said that was slightly more than in August and three and a half times the September historical average.

Timing shaped the picture. The hypoxia DNR observed was confined to Maryland, and Maryland's waters were sampled before the nor'easter. Virginia's waters were sampled afterward, when they had mixed and temperatures had dropped, and those post-storm readings showed reduced hypoxia. The data came from DNR and Old Dominion University.

The jump came late in a season that started mild. The May cruise tied for the smallest volume on record, and early June also ranked first. DNR credited the better early conditions in part to drought, which sent less water into the Bay and with it less of the nitrogen and phosphorus that feed oxygen-robbing algae blooms. The department said the later increase may be partly due to higher temperatures: Maryland's average air temperature for August through September 2026 tied for the fifth-highest on record, the release said.

The spring outlook carried a caveat. When scientists from William & Mary's Batten School & VIMS, FlowWest and the University of Maryland Center for Environmental Science forecast a mild 2026 dead zone on May 19, they cautioned that summer weather such as heavy rainfall, heat waves or extended calm periods could still change its size and duration. That forecast release said researchers would conduct a full assessment of the dead zone's extent and duration this fall. DNR said monitoring results so far align with the seasonal forecast, which predicted Bay hypoxia volumes about 31% below the long-term average.

Crabs, fish, oysters and other Bay creatures need dissolved oxygen to survive. DNR calculates hypoxia volumes each May through October, and more Maryland water-quality data are posted on its Eyes on the Bay website.